SECURITY: Sanitize shell command args

Add validation and sanitization to several code paths.

Bug: 60339
Change-Id: Id124281d21ec730a2e0bbace843dd97194a712b4
Update release notes and version number

Change-Id: I7bb1d944c963dffdb29b96d0dbe64f9a905394f7
Merge branch 'REL1_21' of https://gerrit.wikimedia.org/r/p/mediawiki/core into REL1_21